The present invention is directed to printed wiring boards having through holes and, in particular, to a desmearing process and a solvent used therein for removing resin smear after drilling the through holes.
Printed circuit boards are formed from a layer of conductive material (commonly, copper or copper plated with solder or gold) carried on a substrate of insulating material (commonly glass-fiber-reinforced epoxy resin). A printed circuit board having two conductive surfaces positioned on opposite sides of a single insulating layer is known as a xe2x80x9cdouble-sided circuit board.xe2x80x9d To accommodate even more circuits on a single board, several copper layers are sandwiched between boards or other layers of insulating material to produce a multilayer circuit board.
To make electrical connections between two or more circuit layers on opposite sides of a double-sided circuit board, a hole is first drilled through the two conducting circuit layers and the insulator board. These holes are known in the art as xe2x80x9cthrough holes,xe2x80x9d particularly if they extend through the entire circuit board. Through holes are typically from about 0.05 mm to about 5 mm in diameter and from about 0.025 mm to about 6 mm long. The through hole initially has a nonconductive cylindrical bore communicating between the two conductive surfaces. A conductive material or element is positioned in the through hole and electrically connected with the conducting sheets or layers to complete an electrical connection.
Like double-sided circuit boards, multilayer circuit boards also use holes in an intervening insulating layer to complete circuits between the circuit patterns on opposite sides of the insulating layer, as well as intermediate layers. Unless the context indicates otherwise, references in this specification to xe2x80x9cthrough holesxe2x80x9d refer to these holes in multilayer boards as well, even if they do not literally go through the entire circuit board.
When a through hole is drilled through a double-sided or multilayer circuit board, the act of drilling leaves a smear of insulating material in the barrel of the hole, as well as on the conductive surface. This smear must be removed prior to positioning or depositing the conductive material or element in the through hole if conductive contact between the through hole and the conducting sheets or layers is to be achieved.
Smear removal processes and solvents are known in the art. Several mechanical and chemical desmear methods that are known in the art are described in U.S. Pat. No. 4,601,783, issued Jul. 22, 1986 to Krulik. One common smear removal process utilizes an alkaline permanganate solution to remove the resin smear. U.S. Pat. No. 4,820,548 to Courduvelis et al. describes an alkaline permanganate desmear process that involves three chemical steps. In the first step, called solvent etch, a solvent is applied to attack and soften the resin structure of the smeared material; in the second step, a permanganate oxidizer is applied to remove the swelled resin; in the third step, a neutralizer is applied to neutralize and remove the permanganate from the resin surface. Suggested solvents for use in the disclosed desmear process include alkaline solutions of propylene glycol ethers. Other known solvents for use in permanganate desmear processes include such chemicals as ethylene glycol monobutyl ether, diethylene glycol monobutyl ether, or tripropylene glycol methyl ether. Permanganate oxidizers used in the desmear process include alkaline solutions of sodium, potassium or lithium permanganate. Neutralizers used in the permanganate desmear process are typically oxalic acid or aqueous acidic solutions such as dilute sulfuric acid or hydrochloric acid. Other known neutralizers are acidified stannous chloride, hydroxylamine hydrochloride or formaldehyde.
The known solvents used in permanganate desmear processes work fairly well in removing resin smear from a laminate material extensively used and known in the industry by the designation FR4, which is a fire retardant epoxy resin fiberglass cloth laminate having a glass transition temperature (Tg) of about 140xc2x0 C. In recent years, however, specialized materials, resins and fillers have been used to develop new printed circuit board laminates having improved properties adequate for both state-of-the-art processing and performance requirements. Due to their chemical structure, the specialized resins developed for high performance laminates are very difficult to desmear using permanganate chemical processes. One such specialized resin is an epoxy, polyimide, cyanate ester resin system marketed by Allied Signal under the trademark RCC(copyright) (Tg=160xc2x0 C.). Another high performance resin is BT epoxy, a combination of bis-maleimide triazine (BT) resin and an epoxy resin (Tg=185-195xc2x0 C.). A further high performance resin is polyimide resin, which can have a Tg rangining from 250xc2x0 C. to 300xc2x0 C. or more depending upon the polyimide resin system. Permanganate desmear processes can be used for smear removal of such high performance or specialized resins, but satisfactory smear removal can only be achieved at the highest recommended levels of solvent and permanganate. For many users, such high levels are not practical.
In U.S. Pat. No. 5,985,040, assigned to the same assignee as the present application, there is disclosed an alternative solvent system that provides increased resin smear removal from high performance laminates compared to standard permanganate desmear processes. The alternative solvent comprises a mixture of at least two solvent components, with at least one solvent selected from a first group consisting of gamma-butyrolactone, ethyl-3-ethoxy-propionate, N-ethyl-2-pyrrolidone, N-(2-hydroxyethyl)-2-pyrrolidone, N-cyclohexyl-2-pyrrolidone, and N-octyl-2-pyrrolidone and mixtures thereof, and at least one solvent selected from a second group consisting of N-methyl-2-pyrrolidone, 2-pyrrolidone, tetrahydrofuran, N-ethyl-2-pyrrolidine, N-cyclohexyl-2-pyrrolidone, and N-dodecyl-2-pyrrolidone. The solvents are selected so that the solvent of the first group is different from the solvent or solvents from the second group, and there is a mixture of at least two solvents. It was emphasized in the patent disclosure that better smear removal was achieved with a combination of solvents from the first and second groups than if a solvent from the second group was used alone.
The combination of solvents disclosed in U.S. Pat. No. 5,985,040 is highly desirable for removing resin smears of high performance resins, such as RCC(copyright) and BT resins, and is very useful for removing resin smears from laminate circuit boards that are comprised of combinations of different resin materials, such as RCC(copyright) and FR-4 materials, in different layers. As can be seen from Table B in the patent, the combination of solvents provides a fairly uniform smear removal rate for the different resin materials that make up such combination laminates. One draw back with using a combination of solvents, however, is that the relative concentrations of the solvents in the combination can be difficult to control under production conditions.
It has now been found that, for circuit boards made from a single substrate material, satisfactory smear removal can be achieved using a particular solvent, gamma-butyrolactone, alone. This single solvent is useful for removing resin smears from circuit boards made from FR-4 materials, as well as from high performance epoxy resin materials such as RCC(copyright) and BT resins. However, because the gamma-butyrolactone solvent alone can give uneven smear removal rates for the different resin materials, it is not as effective as the combination of solvents for removing resin smear from laminates made with combinations of resin materials. Nevertheless, for circuit boards made with a single substrate material, utilizing a single solvent provides an advantage over the combination of solvents disclosed in U.S. Pat. No. 5,985,040 because the concentration of a single solvent is easier to control under production conditions.
An object of the present invention is to provide a solvent that provides increased smear removal compared to standard permanganate desmear processes.
A further object of the invention is to provide a process that is useful for preparing a resinous substrate for subsequent metallization.
Another object of the invention is to provide a hole cleaning process for use on high performance circuit board substrate materials, especially materials employing RCC(copyright) or BT resins, that is easily controlled under production conditions.
Another object of the invention is to provide a process for simultaneously cleaning resin smears from the inside walls of through holes and preparing the inside walls for subsequent metallization.
One aspect of the invention involves utilizing an aqueous solution of a single solvent, gamma-butyrolactone, to soften the resin smears on high performance materials, especially those made with RCC(copyright) or BT resin systems.
Another aspect of the invention provides a process for removing resin smears from a resinous substrate which comprises the following steps:
(a) contacting the resinous substrate with a mixture comprising a solvent component and water to soften and swell the resin smear, the solvent component consisting essentially of gamma-butyrolactone;
(b) contacting the resinous substrate with an alkaline permanganate solution for a time period sufficient to remove the softened resin smear; and
(c) contacting the resinous substrate with an aqueous acidic solution for a time period sufficient to neutralize the alkaline permanganate solution and remove substantially all manganate residues.